Because one of these allows you to make predictions, and the other doesn't. Saying "fire has a cause, and I'm going to call it 'phlogiston'!" doesn't tell you anything about fire, it's just a relabeling.

The hypothesis went a little deeper than that. "Flammable things contain a substance, and its release is fire" lets you make many predictions — e.g., that things will burn in vacuum, or that things burned in open air will always lose mass (this is how it was falsified).

I'm not sure I follow, oxidization doesn't predict gaining or losing mass (on any scale like phlogiston would, that is), it predicts an interaction of materials forming a new composite substance. Oxidation doesn't prevent material from being lost or changed in other ways which could cause an overall greater or lesser mass than the original object. What it does predict, however, is that the total mass of all molecules in the equation, once accounted for, will be the same. This is consistent with observation.

If phlogiston has a negative mass, then anything that can burn must gain mass. I don't see any way around it. The theory states that it is a release of negative material, and there is no way to account for it once released.

One thing you would expect to find with phlogiston is an object that was primarily made up of phlogiston, giving it a negative mass. Explosives, for example, clearly have so much phlogiston that it literally rips the object (and anything nearby) apart when released. You would therefore expect all explosives to be relatively light in spite of the original weight of their components.

You could test this with black powder: saltpeter, charcoal, and sulfer each release a certain amount of phlogiston when burned. Combine them and a significantly more phlogiston is clearly released. You would therefore expect more phlogiston to have flowed into the material during the combination of the three objects during the making of gunpowder. However, the weights actually stay quite the same. The observation doesn't bear out the prediction, so the prediction is clearly wrong. If the prediction is wrong, the theory that made it is either wrong outright, or flawed in some way. Since the only prediction phlogiston can make is wrong, then the theory is at the very least flawed in some crippling way, and needs to be completely re-worked.

It's lack of ability to predict expectations is what killed it. You can predict what will happen when you add oxygen to a reaction. You cannot predict what will add phlogiston to a material, thereby allowing it to burn.

A huge example is the sun. It is a giant ball of fire - therefore, a giant ball of phlogiston, or at least a very significant portion of its mass to be made up of phlogiston in order to burn that intensely for that long. So it should have a low mass, possibly even a negative mass. Yet this giant ball of mostly phlogiston is actually the heaviest thing in the solar system by a massive margin.

Phlogiston is incompatible with many, many theories that have been independantly verified. Also, oxygen causing fire is not the theory. The theory is molecules and their chemical interactions, of which oxygen is just one type, and the predictions of oxygen causing most of the exothermic reactions is consistent with all other chemical reactions and is predictable based on rules that are consistent whether a reaction is exothermic or endothermic, among a great many other things. It also predicts which objects will burn and which will not. This same chemical theory leads to atomic theory, which predicts fusion, which has absolutely nothing at all to do with oxygen, yet describes the behavior of the sun very accurately before you even start to measure the sun's output.

The way to test a theory is to predict first, then observe. This is basic science. Phlogiston cannot pass this test, chemical theory can.